Field of the Invention
The present invention relates to powder coating technology which can be used to produce hard, uniform coatings through the heat curing of a powder uniformly applied to a surface. In particular, it relates to the production of a coating composition and to its application to a surface which coating composition, after heating, forms a shiny, reflective metallic appearance. The preferred powder coating is based on aluminum flakes and a thermosetting resin.
Bright, shiny, metallic finishes are desirable in many commercial applications. The major source of these shiny finishes is chemical or electrical plating of metals such as chrome. However, chemical coating processes such as chrome treatments have environmental and economic drawbacks. Chromium is a major source of heavy metal contamination and is expensive to produce. With this in mind, a great deal of attention has been given to producing shiny coatings with powder compositions. Powder coatings are generally produced by mixing a binder with other constituents which can be melt mixed, cooled, and pulverized to provide a powder material that is easily applied to a conductive surface. The powder or surface is then heated to cause the powder coating composition to melt and flow to form a continuous coating.
The powder coating compositions are most commonly applied through electrostatic spray coating or fluidized bed coating. In the electrostatic spray process, the coating composition is dispersed in an air stream before being subjected to a high voltage field in which the particles pick up an electrostatic charge. The charged particles are then attracted to a grounded or charged substrate. The coated substrate is then heated to a temperature sufficient to melt the powder coating and to cause it to flow providing a smooth, even finish.
Fluidized bed coating makes it possible to apply films as thick as about 2.5 mm. In this method, the powder coating composition is fluidized in a fluidized bed by passing a fluidizing gas stream into a vessel containing the powder coating. A substrate is heated in an oven to a temperature above the melting point of the powder coating and is then dipped into the fluidized bed. The powder coating particles contact the hot surface, then melt and flow on the substrate surface. See generally, Kirk-Othmer, Concise Encyclopedia of Chemical Technology, pp. 944-945 (1985).
One attempt to achieve a chrome-like finish has been through the incorporation of aluminum particles or aluminum flake into powder coating compositions. However, mere incorporation of aluminum flake into powder coating compositions has not produced a highly shiny, reflective chrome-like appearance as measured by the high reflectance of conventional chrome electroplating without special and additional processing techniques and additives.
Powder coating composition and processes for the production of such powder compositions are needed which can form a glossy, highly reflective metallic finish that appears to be as shiny as chrome plate.
U.S. Pat. No. 5,045,114 describes a powder product that results from a process of preparing a powder coating material which can be applied to a substrate to form a coated article having a glossy, high reflective, metallic finish. The process comprises milling a resinous binder and about 1 to 12 parts of aluminum particles per 100 parts of binder, whereby the resinous binder and aluminum particles are milled and then separated from both milling media and oversized particle to produce a powder coating composition having a particle size between about 5 to 250 microns. Such powder coating materials, when applied and melt formed on a substrate surface, can exhibit high gloss, high reflectance and a metallic finish. The powder coating compositions can comprise (a) a resinous binder, (b) about 1 to about 12 parts of leafing aluminum particles per 100 parts of binder, and (c) an effective amount of a fluidizing agent per 100 parts of binder which are modified in accordance with the invention. The resinous binder and the aluminum particles are milled and then separated from the milling media and oversized particles to produce a powder coating material having a particle size between about 5.0 and about 250 microns. The aluminum flakes in all examples are provided as a paste of aluminum flakes in mineral spirits.
U. K. Patent No. 1,404,556 titled Particulate Coating Compositions and Coated Articles Prepared Therefrom describes a dry particulate composition comprising a fusible epoxy or acrylic resin and flake particles (selected from titanium nitride flake, nickel sulfide flake, cobalt sulfide flake and manganese sulfide flake, with the flakes embedded in the resin particles or affixed to their surface. Combination with aluminum flake, copper, bronze, brass, chromium and nickel flakes are also shown. The flakes and resin powder are mixed under shear conditions to reduce the size of the flake and to affix the flake to the resin. Pebble mills and any other type of high shear mixer is suggested. Examples show ball milling of the flake and resin particles (Example 1) and melting of the resin and particles in a roll mill (Example 2). A gloss over ninety is asserted to be the result of the process, with Example 2 showing a 20.degree. gloss of 100.
U.S. Pat. No. 5,187,220 describes a thermosetting resin based coating powder containing metal flakes. The powder and flake are mixed at a temperature in the range above the softening temperature of the thermosetting resin, but below the melting temperature of the thermosetting resin, while the powder and flake composition is subjected to mechanical shear forces sufficient to prevent agglomeration of the coating powder particulates.